


Day 2: Reincarnation

by GemmaRose



Series: BNHA Soulmates Week [2]
Category: Star Trek, 僕のヒーローアカデミア | Boku no Hero Academia | My Hero Academia
Genre: Alternate Universe - Star Trek Fusion, Bodyswap, Klingon!Bakugo, M/M, Reincarnation
Language: English
Status: Completed
Published: 2020-12-19
Updated: 2020-12-19
Packaged: 2021-03-10 16:28:11
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,792
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28180161
Author URL: https://archiveofourown.org/users/GemmaRose/pseuds/GemmaRose
Summary: Katsuki should know better than to touch random alien artifacts by now, really.
Relationships: Bakugou Katsuki/Kirishima Eijirou
Series: BNHA Soulmates Week [2]
Series URL: https://archiveofourown.org/series/2064357
Kudos: 27





	Day 2: Reincarnation

Class H planets were, by and large, Katsuki’s favourites. Class Ms were nice and all, and he’d seen some spectacular Class Ks, but nothing beat the rush of knowing the Captain needed something investigated on a Class H. Sometimes the planet was uninhabitable because of something mundane like radiation or a toxic atmosphere, but other times there was carnivorous megafauna or unusual tectonic activity. Not that he cared about tectonic activity in the slightest, but if they were studying tectonics then Eijirou got to beam down too, and he could basically do whatever. Ei’s promotion from security to science had been a surprise, but apparently all those classes he took wherever he went before the Starfleet Academy meant he was qualified, and they kept needing a geologist, so... promotion for Eijirou, and the odd day off for Katsuki.

Today was one of those unofficial days off, the planet marked Class H for something other than giant vicious beasts, so he left Ei to his work and wandered enough of a patrol-like pattern through the long-abandoned ruins of what had once been a village that he had a plausible excuse for leaving the science team alone. The people who’d lived here, the briefing said, had been highly varied in appearance and as far as Starfleet was aware had destroyed themselves due to some unknown factor which had to be determined before they could decide if the planet could be downgraded to Class M and offered up for colonization. Which was all well and good for the scientists, who got to take all kinds of interesting readings Ei would certainly talk his ear off about over dinner tonight, but Katsuki’s job was protection. If there was nothing to protect from...

He poked his head inside what looked like it had once been a jewelry shop, judging by the faded sign over the shattered windows, and motion caught his eye. Nothing alive, he realized after a moment, but a scrap of cloth which had managed to stand up to the years. Huh, he was pretty sure at least one of the nerds would love that. He snapped a few pictures of the fabric and its surroundings as he approached, and when he caught it in his hand it was surprisingly sturdy. Well, it would have to be wouldn’t it, to stand up to the elements this long? He pulled gently, and then had to hop back quickly as the shirt- it was a shirt, black with a bold orange X and holes punched through the chest- made a box fall from the counter and snap open.

The pendant which spilled from the now-open box would’ve been unremarkable in any shop in the Federation, but on the filthy floor of a long-ruined shop on a Class H world it seemed like all he could see. Violently red, the outer edge bore square protrusions like the teeth of a gear, and the center held a mess of woven thread which was either black or a very, very dark red, suspending a fire opal in the precise center. He should shove it back in the box and bring both things back to the science team, tell them about this shop with untouched artifacts, but... just looking at it, all Katsuki could think was how perfect the simple silver chain would look around his husband’s neck. Ei’s birthday was coming up in a few months, and so long as Katsuki didn’t _tell him_ where it came from there wouldn’t be a problem, right?

He stooped to pick it up, and the pendant fit perfectly in his palm, warm to the touch despite the comfortably cool air in the shade of the shop. That was the only warning he had before pain stabbed through his head, his hand spasming tight around the pendant as he fell to the floor.

\--- 

_And all you need is my hair, and his old shirt?_ Eijirou’s voice, high and young and trembling, wrapped around Katsuki in the darkness.

_For now, yes. I will need more to complete it, but that will be later._

_Thank you. Really, I can’t thank you enough._

His chest burned, palm throbbing, and Katsuki jackknifed upright in a bed that was- wasn’t? was- his own. The room flickered around him, like a holodeck session on the fritz, and he clutched his head with a groan as sharp pain bloomed behind his temples again. A pathetic little noise slipped from his lips, and he choked on his next breath as his fingers splayed over smooth skin. The callouses on his fingers were wrong too, but that was nothing compared to the disappearance of his crest. He scrambled out of bed, taking in the room around him, and soon enough his eyes landed on a mirror.

The reflection that greeted him was not his own, but that of a human juvenile. A strong one by human standards, well-muscled and with the scars of a warrior, but the body was not his own. Except, he realized with a hitch of his breath, for the eyes. The eyes were still his own, red as human blood and shaped properly. They looked strange, set in a human face.

A knock came at the door, and he quickly decided that the sleep pants this body wore were enough to be socially acceptable for anyone knocking at his bedroom door, regardless of when in human history he’d found himself. He pulled open the door, and all his breath left him in a rush at the sight of Eijirou. He was human as well, and the crimson hair on the sides of his head had been shorn down to the skin, but what remained in front had been fashioned into a mimicry of his husband’s sensory horns.

“Ei.” he gasped, and caught Eijirou by the wrist, dragging him inside to shut the door, this body’s muscle memory turning the lock blindly even as he fumbled to grip the youthful human version of his husband by the back of the neck. It was strange, being of a height with him instead of half a head shorter, but it sure did make reeling him in for a kiss easier. Eijirou’s lips felt strange against his, but his husband kissed back with clumsy enthusiasm and they didn’t part until his current body’s lungs demanded air.

“Wow.” Eijirou beamed, eyes sparkling and teeth- huh, apparently he’d kept his teeth, because Katsuki knew for a fact humans didn’t have teeth like that. Nice. “Guess you’re feeling better, Bakugo?”

Katsuki jerked back, heart pounding against the inside of his chest as he took in the human incarnation of his husband again. Young, the same age as the human body he wore, familiar but not- not _his_. “You’re not him.” he said, dread settling heavy over his heart.

“Bakugo, what do you mean?” Not Eijirou frowned, and _damn_ but it was a good likeness. With the hair mimicking his sensor horns, if he squinted a bit he could’ve been looking at one of his husband’s childhood photos. “I’m me.”

“Sure, yeah, but you’re not him, you’re not my Ei.” Katsuki’s hand spasmed, and he looked down to see his fingers curled around an invisible mass for just a moment before they closed against his palm. The pendant. Ah, _baktag_ he was gonna get an earful about that when he got back to the ship. It was definitely one of the informal rules at this point, Don’t Touch Random Jewelry. He’d even _noticed_ how it drew his attention, before it compelled him to pick it up. So much for a quick present.

“What do you mean, Bakugo?”

“That’s not my name.” he spat, drawing himself up as tall as he could in this pathetic human body. Regardless of what body he was in or how ill-dressed he was, he was a professional and he was damn well going to act like it. “I am Kirishima Katsuki, security officer aboard the Federation vessel Mamoru. My last known location was the Class H planet Egnillion III, stardate 1208.5.” he crossed his arms and lifted his chin slightly, narrowing his eyes at this humanoid who so resembled his husband. Possibly a telepath of some description, considering they had known his old clan name. That or they were working with whoever had done this to him, in which case he was going to find out right quick how far he could push this body in a fight.

“Who are you, where am I, and what day is it?”

“Uh, Kirishima Eijirou, you’re in the UA dorms, and it’s uh, Sunday?”

“I asked for _your_ name.” he snarled, and something exploded inside the cage of his fingers.

“Whoa, whoa! No need to bring quirks into this, okay?” Not Eijirou raised his hands, palms out and fingers spread in the universal gesture of peace. “Ba-” Katsuki bared his teeth with a snarl, and the p’tok corrected himself. “Katsuki, something’s wrong. Do you want to go to Recovery Girl, or should I ask if she can come here?”

Recovery Girl could only be what they called their medics, though it was a strangely singular title. Maybe they only had one on site? He tucked that information away for later. He was without weapons, in a body with limits he did not know, surrounded by potential hostiles in an unknown location. Klingons had something of a reputation among the Federation as one-soldier armies, but even if he’d been in his own skin Katsuki wouldn’t’ve taken these odds. “Alright, bring me to Recovery Girl.” he squared his shoulders, and was surprised when Not Eijirou didn’t furnish any sort of cuffs or other manner of restraint. There must be incredibly tight, well-trained security on site if they didn’t feel the need to restrain him.

Maybe they were counting on him not being able to attack someone who looked so much like his husband? Poor choice on their part, this humanoid was only a pale visual shadow of the mecoth he’d fallen for. If he had to, he could do some serious damage. Picking a child was clever though, he would be hard pressed to go straight for lethal damage on a juvenile opponent.

Not Eijirou pushed a shirt into his hands, which he pulled over his head without examining, and Katsuki followed his captor out into the hallway. They seemed to be in a dormitory, with an elevator at one end of the hall and stairs at both. The ground floor- ground, they were either planetside or on a massive space station meant for long term habitation- continued the dorm appearance, with a large common room and a kitchen. Not Eijirou handed him a pair of shoes, flimsy things he immediately disliked on principle, and Katsuki trailed after him out the door. Planetside, he decided as the smell hit him. Even the nicest long-term space stations didn’t import this much dirt for habitation zones, let alone ones where you’d stick a prisoner.

The building Not Eijirou led him to was tall, shiny with windows, and barely guarded. Katsuki was starting to get the feeling he could actually fight his way out of here, if he got back into his proper body. Of course, then came the issue of getting himself or a message offworld, but first things first, his body. The medical office he was led into did in fact seem to be staffed by a single human-looking elder. Short and stooped with age, their hair was pulled up in a tight bun and they gave a nod of acknowledgement on seeing him and his captor.

“Have a seat, Kirishima. Bakugo, on the bed.”

Katsuki stalled, looking at Not Eijirou and then at the bed. “Something’s wrong with his memory, Recovery Girl.” his captor said, grabbing him by the arm with surprisingly little force and guiding him forwards towards a bed with collapsible railings. “Tell her what you told me, Katsuki.”

“My name is Kirishima Katsuki.” he said firmly. “I am thirty five Federation Standard years old, and serve as a security officer about the Constitution Class Federation vessel Mamoru. I passed out on the Class H planet Egnillion III on stardate 1208.5, and woke up in a room on this... campus.” he crossed his arms and set his jaw. Medics, he’d learnt from long experience, tended to have more answers than one expected, and usually far less training in not getting it wheedled out of them. “Where is my true body, why and _how_ have I been placed in a human child, and why have both of you called me by the name of my birth clan?”

“Ah.” the Recovery Girl said, adjusting her glasses. “Kirishima, run along and fetch Aizawa for me, will you? I think I know what’s done this.”

“You _think_?” Katsuki’s hands fell to his sides, his fingers curling into claws as Not Eijirou fled the room. Small explosions tickled his palms, and the elderly- doctor? nurse? if they were the only one, probably a doctor- gave him a scathing look.

“Yes, I think. I’ve not run all the tests to know precisely what happened, yet, but I can make a good guess.” the doctor gestured to the bed. “Sit, and do try not to burn the sheets.”

Katsuki curled his hands into fists, but did as the doctor ordered while they boosted themself up onto a stool.

“Now, nobody here at UA has the ability to swap people between bodies, but there are quirks which can do just that. ”

Katsuki wondered for a moment what a quirk was, in this context, but quickly dismissed it as unimportant. What mattered was he wasn’t here by the volition of anyone in power at this facility, which meant whoever’s body he was inhabiting _was_ supposed to be here, and they would want to bet both of them back where they belonged as quickly as possible. “Alright, and I’m guessing Aizawa is someone who can reverse whatever’s been done to me?”

“He’s going to be the first attempt.” the doctor nodded. “You’re currently in the body of one of the students here, Bakugo Katsuki. The boy who brought you to me is Kirishima Eijirou.” ah, that did explain a few things. Namely, well, the names they’d used for him. The doctor smiled, in the knowing way of meddling elders, and Katsuki got the feeling that were he not still ready to defend himself he would’ve gotten poked. “I have a feeling young Bakugo will be subjected to some very personal questions, once he’s been returned.”

“Please.” Katsuki scoffed. “That p’tok isn’t going to make a single move. You give the idiot who owns this body a good whack and tell him to get his heart settled, if your Kirishima is anything like my Ei, there’s no better for Bakugo to have at his side against their enemies.”

Damn, it was weird talking about his young, alien self in third person. The doctor laughed, and hopped down off their stool to come over and pat his knee. “We will get you back to your husband, I promise.”

“What did he do this time?”

“Got hit by a body swap quirk of some sort.” the doctor replied, not even looking at the unshaven, clearly exhausted human who’d come in the door. “Try to nullify it, please.”

“Of course, Recovery Girl.” the man nodded, and Katsuki’s hand spasmed around a pendant that wasn’t there, then suddenly was. The room flickered, and as suddenly as he’d woken in the wrong body he was returned to his own. His head still hurt like absolute baktag, but at least it was his own damn head again. He pushed himself to his feet, used the tip of his phaser to nudge the pendant back into its box, and gathered the box and filthy shirt in his hands.

He hadn’t been out for long at all, judging by the local sun’s lack of notable motion in the sky, and when he got back to the nerd squad plus Ei he found some of them still working. “I found intact fabric and a box with something shiny.” he announced, and half the team was on him at once, pulling the shirt and box from his hands to go fawn over. “Here, I took some pics of where I found them.” he selected the relevant files and sent them over to one of the workstation computers.

“Well, I think you just made sure we’re coming back again tomorrow.” Ei chuckled, walking up and slinging an arm around the small of his back. “Hope your day was as boring as mine.”

“I’ll tell you about it over dinner.” he promised, and leaned up to steal a kiss. Gods, it was good to be home.

**Author's Note:**

> Klingon glossary! Baktag is shit/garbage, P’Tok is “spineless human child”. Mecoth is not a Klingon word, just a random name chosen for trek!Kiri’s species. And fun fact, trek!Kiri’s full name is Bakugo Eijirou, because Alien Marriage Customs!
> 
> Apologies to any not-logged-in readers, but due to an ex who refuses to leave me alone I have had to disable anon comments. Kudos are still open though, and if you want to scream (or would like me to write a fic for you) come check me out on Pillowfort! No account required to get my discord, and I'm always happy to chat. [[Link](https://www.pillowfort.social/GemmaRose)]


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